Electrophotographic copiers and printers create a loose toner image on paper. The most common method of fixing the toner image to the paper is to feed the paper and image between a pair of fusing rolls. The toner image is then fixed to the paper by a combination of heat and pressure applied to the image by the rolls.
The toners used have a thermoplastic component which softens in response to heat. Such systems are effective within a narrow temperature range which must be high enough to soften a thermoplastic having a glass transition temperature high enough to have good keeping characteristics. At the same time the fuser cannot be so hot as to char the paper or risk starting a fire. Not surprisingly, quality temperature sensors are used in all such systems both to control the heat being applied to the fuser as the temperature varies but also sensors to prevent damage if the system severely overheats.
Temperature sensors can be placed, and are placed, in the core of an internally heated fusing roll, on the surface of the fusing or other roll and in the air surrounding either roll. For a number of reasons, it is especially desirable to know the temperature of the surface of the fusing roll. This is especially true in systems in which the fusing roll itself is externally heated, for example, by contact with heating rollers.
Sensing the temperature of the surface of any of the rolls in a fuser has created problems for the industry. Present approaches include placing a commercial thermostat in direct contact with the surface. Typical thermostats used for contact sensing include a bimetal disc whose direction of curvature changes from convex to concave according to a threshold temperature and a mechanical switch responsive to such change in curvature. A metal housing is positioned between the bimetal disk and the roll. If the metal housing is not in contact with the roll, response through the air gap is quite slow. Even with the metal housing in contact, temperature response is relatively slow and the additional problem of wear between the fusing roll and the housing of the thermostat is present.
Attempts at reducing the wear include placement of a carbon-shoe between the thermostat and the roll which carbon-shoe is shaped to the contour of the roll and conducts the heat reasonably well to the thermostat.
Unfortunately, all of the above devices show a slow response to changes in temperature. That is, if the device is being used as a safety sensor, a rapid rise in the temperature of the surface of the roller will not be sensed by the safety sensor until a certain length of time thereafter. This forces the setting of the thermostat at a relatively low temperature in order to be sure to cut off before the paper chars or a fire begins. Such a low temperature is unfortunately close to the operating temperature of the fuser itself providing very little margin for ordinary fluctuations and forcing use of lower temperature fusing materials than desirable.
The problem of slow response time in responding to a change in the surface temperature of a roll used in fusing is generally troublesome for the industry with conventional internally heated rolls. However, when a fusing roll is heated externally, the problem is magnified. An externally heated fusing roll can be heated rapidly in response to an appropriate signal. This characteristic is quite advantageous for control of the fusing temperature itself. However, it also means that the temperature can reach a danger point quite rapidly if something goes wrong with its control system. The safety cut off circuitry therefore must have a fast response time to prevent charring of paper or a fire.